What is "downgrading a system"?

In Debian world, it means to move to an older distribution (typically, to move from unstable to testing). Do not expect to get anything stable by downgrading!

Is it supported?

Short answer: No, it isn't supported.

Long answer: It isn't supported because 1. The packages' installation scripts (postinst...) are designed to handle upgrade only. 2. The installation tools are designed to replace older versions of packages by newer versions.

So, what's the supported way?

Make a backup of your system before you upgrade, so you can restore your backup to "downgrade" it!